Approximately 80% of the energy used around the world is generated by the conversion of fossil fuels. This is associated with significant CO2 emissions, which are considered one of the causes of global warming. A first goal is therefore to reduce, or completely avoid, CO2 emissions directly at the power producing plants.
In principle, three routes are possible for separating CO2 in power plant processes using fossil primary energy carriers.
a) Separating After Energy Conversion:
Using complex apparatuses and chemical treatment, CO2 having low concentrations is removed from the low-temperature waste gas flow of the energy conversion plants (separating task: CO2/N2)
b) Oxygen Combustion
The use of pure oxygen instead of air as the oxidizing agent for the combustion of gas or carbon results in a lesser quantity of highly CO2-enriched waste gas having low nitrogen fractions, from which the CO2 can generally be removed considerably more easily than in process a). The disadvantage is that pure oxygen must first be obtained (separating task, air separation: O2/N2). While cryogenic air separation is available on an industrial scale, it is very cost-intensive depending on the purity level of the separated O2 (95% to 99.5%).
c) Decarbonizing Prior To Energy Conversion:
Here, the carbon is removed from the fossil fuel prior to the actual combustion process by convertion of the fuel, notably carbon, into CO2 and hydrogen gas by partial oxidation or reformation (separating task: CO2/H2), and the combustion of hydrogen. The CO2 can be washed out using physical or chemical washing solutions. This is also an easier method for separating the CO2 from the gas mixture than the process described in process a) because, here again, considerably higher concentrations and pressures are present for the CO2.
All the concepts described above result in a considerable reduction of the thermal efficiency and require complex apparatus, which therefore make these energy conversion methods having reduced CO2 output more cost-intensive.
Until now, neither solid adsorbents, nor porous membranes, nor zeolite beds or membranes have been able to effect such gas separation in a suitable manner, cost-effectively, and on an appropriate scale.
A potentially suitable method, which is associated with considerably lower efficiency losses, is gas separation by way of ceramic membranes. Ceramic membranes have high chemical and thermal stability and can be used in all three power plant routes. Existing ceramic membranes, however, have insufficient permeation or separation rates or are not stable under process conditions.
The permeation rate constitutes the volume flow per unit of time of the permeating component, relative to the membrane surface [ml/(cm2 min)]. The selectivity is described with what is referred to as the separation factor, which is derived from the ratio of the permeation rate to the gases to be separated, and is infinite for dense but oxygen-semipermeable membranes.
In addition, with respect to membranes, a differentiation is made between bulk membranes and asymmetrical membranes. While a bulk membrane (monolithic membrane) has a single material layer, an asymmetrical membrane has a layer design comprising at least two different layers, a separating layer and a porous support layer.
All monolithic membranes that have been developed for the above fields of application with layer thicknesses of 0.5-1 mm, however, have insufficient permeation rates and/or insufficient stability with respect to the thermochemical or thermomechanical demands.
With regard to the asymmetrical membranes, a differentiation is made between so-called integral asymmetrical membranes, in which the separating layer and support layer comprise the same material, and the composite membranes, in which the multi-layer design is achieved by applying a gastight separating layer onto a previously manufactured (micro)porous support layer using a separate step.
It is conceivable to produce asymmetrical membranes having separating layer thicknesses of less than 100 μm from materials that have maximum permeation rates, such as Ba0.5Sr0.5Co0.8Fe0.2O3−δ). However, given the extraordinary thermal and chemical expansion characteristics of these materials, they necessarily require a carrier that is made of the same material as the separating layer. This material, however, frequently has lower chemical and mechanical stability and is also relatively expensive.
For this reason, selecting a suitable material for use in a separating membrane and in the support layer thereof for a specified separating problem is one of the greatest current challenges. The search is for a material which not only has high permeability, high selectivity, and high chemical stability, but additionally exhibits the necessary thermomechanical properties that make it possible to use this material to produce a defect-free, thin, yet elastic film for a membrane.
Presently, both planar and tubular concepts are available for gas separation by way of membranes, wherein a graded layer structure is generally present. Starting from a mechanically stable, macroporous substrate having pore diameters of 50 to 100 nm, one or more mesoporous (50>dPore>2 nm) or microporous (dPore<2 nm) layers are applied using different methods. For example, approaches are available for separating H2/CO2 and CO2/N2 by way of microporous membranes comprising silica (SiO2), TiO2 and/or ZrO2. Dense ceramic mixed conductors, which generally have a perovskite structure (ABO3−δ where A=La, Pr, Ba, Sr, Ca and the like, and B=Co, Fe, Mn, Cr, Ti, Ni, Cu and the like, or mixtures thereof), are used for oxygen separation from the air.